1. Field of the Invention
The present invention relates to a gas turbine nozzle and, more particularly, to a gas turbine nozzle made of a nickel base alloy.
2. Description of the Prior Art
The nozzles of a gas turbine is subjected to a corrosive gas of a high temperature, during operation of the gas turbine. This corrosive gas is a product of combustion of a fuel such as heavy oil, and is heated to a high temperature of around 1,100.degree. C. This gas contains various highly corrosive elements such as sulfur, sodium, kalium and the like. It is, therefore, necessary that the gas turbine nozzle be made of a material having a high corrosion resistance.
Also, the nozzle is subjected to heating and cooling alternately and repeatedly, due to repetitional operation and suspension of the gas turbine. Therefore, another requisite for the material of gas turbine nozzle is to have a high resistance against thermal shock and thermal fatigue.
Conventionally, cobalt base alloys have been used as the material of the gas turbine nozzle. A typical one of these cobalt base alloy has a chemical composition constituted by 0.28 wt% carbon, 10 wt% nickel, 30 wt% chromium, 1.8 wt.% iron, 7 wt% tungsten, 0.014 wt% boron and the remainder cobalt.
The conventional gas turbine nozzle of the cobalt base alloy is fabricated by a precision casting and is subjected, after the casting, to a heat treatment, i.e. solution heat treatment and aging treatment. The gas turbine nozzle made of a cobalt base alloy exhibits a superior corrosion resistance against the corrosive combustion gas, as well as various advantages such as high creep rupture strength, large rupture strength, large reduction of area upon rupture and small aging embrittlement. On the other hand, however, the gas turbine nozzle made of a cobalt base alloy is liable to exhibit a cracking due to the repeated heating and cooling, requiring frequent repair by welding. The life of the gas turbine nozzle made of a cobalt base alloy, therefore, has a short life, even by the repeated repair by welding. In addition, the cost of production of the gas turbine nozzle is inevitably raised due to the use of expensive cobalt.